Both pages are pretty small and refer to applications in the Graphics menu.

Remove the 'Use Tails' categories and create new ones:

- First steps with Tails
- Connect to the Internet anonymously
- Encryption & privacy
- Work on sensitive documents
- Advanced topics

Each one corresponds to a topic specific intro page, eg. first_steps.mdwn, and a
category index, first_steps.index.mdwn, which is in turn inlined in doc.mdwn and
first_steps.mdwn to avoid duplication.